Contact lenses are commonplace today. Individuals with minor vision ailments can typically acquire and use these lenses in place of prescription eye glasses. This is not so true, however, for individuals having poor vision, or for those presbyopic persons requiring bifocal glasses. For example, presbyopic individuals can opt for bifocal contact lenses, but may find it difficult to maintain focus during eye movement, or only see clearly at one viewing distance.
The need to develop more versatile lenses has led designers to pinhole contact lenses. These lenses utilize theories of pinhole imaging, commonly understood in optics as a method to reduce geometrical aberrations, e.g., astigmatism, spherical aberration, and coma. By restricting a person's vision to a small "pinhole" aperture, visual deficiencies are greatly reduced, or even effectively removed. Unfortunately, the utility of this technology has met with limited success. For instance, while the classic pinhole lens device produces an enhanced depth of focus, which increases with smaller aperture, smaller apertures typically result in degradation of the quality of the image due to diffraction, among other things. The denser the opacity of the region surrounding, i.e., defining, the pinhole lens also affects the contrast of the focused image. If the region surrounding the pinhole is allowed to pass some light, such as to improve acuity under low light conditions, the image contrast is degraded.
"Multiple Focal Contact Lenses," as described in U.S. Pat. No. 3,794,414, was one attempt to develop small-aperture contact lenses. This approach combined a pinhole-like aperture with radial slits and scalloped masking regions on a contact lens to supposedly correct both peripheral vision and the effects related to decentered contact lenses. It was considered that some disadvantageous diffraction effects were introduced by this type of design.
In addition, it is important to consider the normal functioning of the human pupil. A large drawback in pinhole imaging is energy starvation. Small-aperture pinhole lenses improve image quality, but at the same time block significant amounts of light energy from reaching the retina. Under dim lighting conditions, a human pupil normally dilates. Without proper consideration, a small-aperture contact lens would place a person into equivalent darkness, even though the lighting is only dim or low.
Pinhole correction together with the normal functioning of the human pupil is considered in U.S. Pat. No. 4,955,904, which presents an intraocular lens surgically implanted within the eye. The patent, entitled "Masked Intraocular Lens and Method for Treating a Patient With Cataracts," affords cataract patients some form of vision correction through surgery. The intraocular lens is masked to form a pinhole that accommodates the function of the human pupil under different lighting conditions. But, intraocular lenses have operational and other drawbacks. They are not contact lenses; surgery is required and the lens must be permanently implanted with precision through the use of man-made loops. Furthermore, because of material requirements for implantation, these impenetrable lenses can transmit little or no oxygen, a feature widely available in contact lenses. Contact lenses, in addition, are conveniently installed and removed by the wearer, and are held in place on the eye through tear tension.
U.S. Pat. No. 5,245,367, entitled "Annular Mask Contact Lenses," describes a lens with a small pinhole-like aperture surrounded by an annular mask. The central aperture created by the pinhole, corrects the patient's vision, while the lens body outside the annular mask enables the patient to view both near and far objects comfortably, under differing light conditions. The lens body described in the '367 patent sizes the annular mask to allow the dilated pupil to extend beyond the mask radius, to thereby allow more light to pass under low-light conditions. Additionally, optical (e.g., corrective) power is disclosed for both the aperture area and the area outside of the mask; but not within the mask region.
Despite these and other advances, pinhole-type contact lenses with improved corrective ability are desirable.